As per the advice from Ricky BeamRicky Beam I opened a Cisco TAC case for this issue, just to see what the official word is:
Hi Mark,
thanks. At this time, the ASA does not have native IPv6 PPPOE support. I have not seen it on any future roadmap. You may want to contact your local cisco team to see if they can submit a request by putting in a business case for you.
(emphasis mine). So no, there is no way to do IPv6 on an ASA using a PPPoE connection, and it doesn't look like it is coming any time soon. I guess they expect you to put a proper router infront of the ASA instead.